Third Generation mobile communication systems are modern systems designed for multimedia communications. Using such systems, person-to-person communications can be established which include high quality picture and video images. Access to information and services on public and private networks in enhanced form can also be achieved by the higher data rates and improved data handling capabilities available using such systems.
WCDMA (Wideband Code Division Multiple Access) technology has emerged as the most widely adopted air interface technology for Third Generation mobile technology. Such technology is based upon protocols defined in a number of industry standards. A number of the important international standards have been defined as 3GPP (3rd Generation Partnership Project) standards by ETSI (the European Telecommunications Standards Institute).
In a WCDMA system, all users employ the same carrier frequency in a given channel and all can transmit or receive simultaneously. In order to be able to distinguish between different users, each transmitting user in each channel uses a code sequence which is unique, the ‘scrambling code’. Each channel carrier has a large bandwidth (e.g. 5 MHz), and transmitted user information bits (symbols) for radio communication are spread over a wide frequency bandwidth of the channel by multiplying the user data bits with a spreading code comprising a sequence of ‘chips’ or rectangular pulses having a regular chip length.
In a receiver for use in a WCDMA system, the Channel Profile Acquisition (CPA) stage is a very important stage for decoding a received signal. Such a stage includes a processor known as a path searcher. The purpose of this processor is as follows. In wireless communication, signals may travel along many different propagation paths before reaching a receiver. Since each path has a different length and the signals travel at the same speed, there is multiplicity of arrival times at the receiver for the signals from the various paths. The signals at the different arrival times are replicas of one another differing only in amplitude and phase. The path searcher finds the signals from different paths received at different arrival times.
As defined by 3GPP, the path searcher in one known form may be based on the Primary Common Pilot Channel (P-CPICH) standard in which the transmitted signal may be considered as a P-CPICH signal. According to the P-CPICH standard, each communication frame has a duration of 10 msec in a common timing protocol of the communication system, and each frame contains 15 slots. In each slot, ten complex symbols of 1+j are transmitted. Each symbol is expanded into chip level by a fixed spreading factor of 256 with a spreading code of “all ones.” P-CPICH signals carry data (user communicated data), spreading code, and scrambling code. However, data is in the form of the complex symbol 1+j and the spreading code consists of 256 ones, so the only valuable information extracted from P-CPICH signals is therefore the scrambling code. The path searcher performs a scrambling code correlation with a received signal at each time position of a given set of possible positions to select the positions giving the strongest correlation results in terms of correlation energy.
In order to reduce processing complexity of a processor serving as a path searcher, a procedure known as a double dwell search may be employed. Such a procedure operates as follows. In a first dwell or first stage of processing in the procedure, a short duration correlation (with a given scrambling sequence) is carried out in each of a complete set of possible signal time positions for a given channel. A sub-set of the time positions showing the strongest correlations is selected for further processing in a second dwell or second stage of processing in the procedure. In the second dwell, a longer duration correlation is carried out for each of the time positions selected by the first dwell to select an even smaller sub-set of the time positions for further processing.